One important trend of integrated circuit technology is the scaling down of metal-oxide-semiconductor field effect transistors (MOSFETs) for a higher integration degree of devices and reduction of manufacturing costs. However, it is well known that the reduction of MOSFET sizes will incur short channel effects. As the sizes of a MOSFET are scaling down, the gate of the MOSFET has a smaller effective length, and the proportion of charges in the depletion region which is actually controlled by the gate voltage becomes smaller. Consequently, the MOSFET has a reduced threshold voltage with a reduced channel length.
In a MOSFET, it is desirable to increase the threshold voltage of the device so as to suppress short channel effects. On the other hand, it may also be desirable to reduce the threshold voltage of the device so as to lower the power consumption, for example, in an application with low voltage supply, or in an application using both P-type and N-type MOSFETs.
Channel doping is a known approach for tuning the threshold voltage. However, when the threshold voltage of a device is increased by increasing the doping concentration in the channel region, carrier mobility decreases, and device performances deteriorate. Moreover, the highly-doped ions in the channel region may compensate the ions in the region where source/drain region meets the channel region, which decreases the doping concentration in said region and increases the device resistance.
It is disclosed in “Scaling the Si MOSFET: From bulk to SOI to bulk”, IEEE Trans. Elect. Dev., Vol. 39, p. 1704, June, 1992 by Yan et al. that a ground plane, i.e. a backgate being grounded, may be provided under the buried oxide layer for suppressing the short channel effects in an SOI MOSFET.
However, the SOI MOSFET with a grounded backgate mentioned above is still not able to meet the requirement for threshold voltage while the channel length of the semiconductor device continues to shrink.
Therefore, it is still desirable that the threshold voltage of the semiconductor device can be adjusted in a controllable manner without increasing the doping concentration in the channel or deteriorating the performances of the semiconductor device.